MySQL 8.0

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create table foo (a text, b text);
create table bar (c text ,d text);
insert into foo () values ();
insert into bar () values ();

update foo cross join bar set a='a', b='b', c='c', d='d';
update foo set a=concat(a,b),b=concat(a,b),a=concat(a,b);
select * from foo;

Rows matched: 2  Changed: 2  Warnings: 0

Rows matched: 1  Changed: 1  Warnings: 0

a
b

ababb
abb

update foo cross join bar set a='a', b='b', c='c', d='d';
update foo cross join bar set a=concat(a,b,c,d),c=concat(a,b,c,d),b=concat(a,b,c,d),d=concat(a,b,c,d),a=concat(a,b,c,d);
select * from foo cross join bar;

Rows matched: 2  Changed: 1  Warnings: 0

Rows matched: 2  Changed: 2  Warnings: 0

a
b
c
d

abcdabcdbcdcd
abcdbcd
abcdabcdbcdcdabcdbcdcd
abcdabcdbcdcdabcdbcdcd

update foo cross join bar set a='a', b='b', c='c', d='d';
update bar cross join foo set a=concat(a,b,c,d),c=concat(a,b,c,d),b=concat(a,b,c,d),d=concat(a,b,c,d),a=concat(a,b,c,d);
select * from foo cross join bar;

Rows matched: 2  Changed: 2  Warnings: 0

Rows matched: 2  Changed: 2  Warnings: 0

a
b
c
d

ababcdababcdd
ababcdababcdd
abcd
ababcdd